Restoring iphone 4s to original settings?

I'm selling my phone to envirophone as its not in a good way and will be happy with anything for it. I'm currently wanting to restore the phone but it won't work as find my iphone is on still.

The problem with this phone is that i don't get service or Wi-Fi and the top buttons broken. Yep its awful i know. I have deleted the account for find my iphone on the icloud website but it still doesn't recognize that its been taken off.

any ideas?

You have an iPhone 4S,
the phone is been taken off your account. Good.
Your intention is to sell it to a phone broker - envirophone.

I just checked the web site, they'll offer £91.06 for a good 4S. (~$140 USD)
Less for a broken, guessing £60.00 for a broken power button (the part is cheap, but the labor is a lot because it's very difficult).

You're trying to reset it, but the power/lock button doesn't work.

To RESET:
First, you can wake up the phone by plugging in the charging cord. Your HOME button should get you to the main screen
Next, pick Settings icon
Pick "General" menu
Scroll to the bottom; the Reset menu will be there.
Select "erase all content and settings"
The iPhone should prompt you "Really?"
It will erase all content and reset everything according to the currently installed IOs.

Discussion:
The power button cost is about $10 max with shipping.
The labor is about 1 hour. If you can fix this your self, you can sell it for much more.
If you have a friend that can do it for you, pay them £20 or so, then check the local classifieds. I'm guessing it's worth about £150 - that's a lot of money saved.
There are many on-line videos that you can watch. Be patient, methodical and don't make a move without first KNOWING what you're about to do.